EFFECTS OF VELOCITY RATIO ON MIXING LAYER 3-DIMENSIONALITY

Three-dimensionality in transitioning mixing layers, in the form of st reamwise vortical structures, is now well established and understood. In the present investigation, the effects of velocity ratio, or more p recisely of velocity difference across the layer, on the near-field ge neration and development of three-dimensionality in two-stream mixing layers have been investigated for the first rime. Detailed time-averag ed velocity measurements were obtained using a single rotatable cross- wire probe on fine cross-plane grids at four streamwise locations in t he near-field region. The effects of five velocity ratios, ranging fro m r = 0.5 to 0.9, were investigated by keeping the velocity of one str eam constant while varying that of the other. The results indicate tha t the mixing layer three-dimensionality is strongest for the mixing la yer with the lowest velocity ratio. The peak mean streamwise vorticity levels decrease with increasing velocity ratio at all the streamwise locations investigated. However, the rate of decrease is highest at th e most upstream station and it decreases with increasing downstream di stance. The mean streamwise vorticity contours at the first station sh aw that their distribution is quite similar at the different velocity ratios, implying that the generating mechanism and initial disturbance fields remain unchanged with velocity ratio. However, it is conceivab le that the amount of stretching in the braid regions due to the spanw ise vortical structures is affected by velocity ratio. As the velocity ratio decreases land the velocity difference across the layer increas es), stronger spanwise rollers are generated which increase the amount of stretching in the braid regions and hence the streamwise vorticity is amplified. The splitter plate wake has a lasting effect on the thr ee-dimensional structure of the mixing layers at the higher velocity r atios, The presence and effects of the wake are clearly seen in the me an velocity, mean streamwise vorticity and Reynolds stress contours. ( C) 1998 Elsevier Science Inc. All rights reserved.